find_package: print error if an invalid CONFIGS name is used

In commit 41c2895b (Added version support to Config mode of find_package
command, 2008-01-28) the error message was computed but was not
reported.  Add the SetError call to report it.

Source/cmFindPackageCommand.cxx

@@ -538,6 +538,7 @@ bool cmFindPackageCommand
         e << "given CONFIGS option followed by invalid file name \""
           << args[i] << "\".  The names given must be file names without "
           << "a path and with a \".cmake\" extension.";
+        this->SetError(e.str().c_str());
         return false;
         }
       this->Configs.push_back(args[i]);